Charge pumps use a switching process to provide a DC output voltage larger than its DC input voltage. In general, a charge pump will have a capacitor coupled to switches between an input and an output. During one clock half cycle, the charging half cycle, the capacitor couples in parallel to the input so as to charge up to the input voltage. During a second clock cycle, the transfer half cycle, the charged capacitor couples in series with the input voltage so as to provide an output voltage twice the level of the input voltage. This process is illustrated in FIGS. 1a and 1b. In FIG. 1a, the capacitor 5 is arranged in parallel with the input voltage VIN to illustrate the charging half cycle. In FIG. 1b, the charged capacitor 5 is arranged in series with the input voltage to illustrate the transfer half cycle. As seen in FIG. 1b, the positive terminal of the charged capacitor 5 will thus be 2*VIN with respect to ground.
The generic charge pump described above will provides an output voltage that can be no more than twice the input voltage VCC. U.S. Pat. No. 5,436,587, the contents of which are hereby incorporated by reference, discloses a charge pump having a voltage adder stage followed by a plurality of voltage doubler stages, wherein the stages are cascaded such that output voltages considerably higher than twice VCC may be obtained. While the voltage adder stage uses just one capacitor per output voltage signal, the voltage doubler stages require 2 capacitors for each output voltage signal, thereby increasing manufacturing costs. Replacing all the voltage doubler stages with voltage adder stages, however, would increase the series resistance substantially.
Accordingly, there is a need in the art for efficient charge pumps that require just one capacitor per stage.